This invention relates to methods and apparatus for controlling the initiation and completion of combustion in combustible mixtures. The invention has particular application and is directed to the photochemical control of the initiation and completion of combustion, particularly in fuel/oxygen/diluent combustible mixtures.
Conventionally, combustible mixtures are ignited by raising the temperature of a zone within such a mixture to the thermal ignition point, so that ignition is initiated in the zone and is then propagated throughout the mixture. Thermal ignition temperatures of conventional mixtures are relatively high, and at such high temperatures pollutants are formed. Further, the creation of pollutants in the normal mixture range has been found to decrease with decreasing proportions of fuel in the mixture, so that fuel-lean mixtures generate less pollutants.
The present invention involves the appreciation that the irradiation of a combustible mixture with photon energy in the ultraviolet range preconditions the mixture by the photodissociative creation of a combustion intermediary species. For example, in the case of a fuel/oxygen mixture, additional oxygen atom concentrations are photochemically created far in excess of those existing at the temperature of the mixture. Such concentrations of combustion intermediary species in the mixture result in the effective lowering of the thermal ignition temperature of the mixture and also permit combustion with greatly reduced proportions of fuel and at greatly reduced pressures. Further, such ultraviolet irradiation has been found to affect the reaction front propagation velocity and decrease ignition delay. All of these factors enhance the combustion process and reduce pollutants generated.
Photochemical control of the combustion process is preferable to the conventional control by spark or other sources such as glow discharge, exploding wire, hot wire, and the like. In particular, initiation of the combustion process by these conventional devices does not readily offer a means for controlling the rate of combustion or flame propagation speed as does photochemical control.
In the present invention, the photoconductive creation of a combustion intermediary species may be in a concentration below that required for the photochemical initiation of combustion at the temperature of the mixture but above the concentration of the species that would exist at the thermal ignition temperature of the mixture. In this fashion, the mixture is preconditioned so that the thermal ignition temperature is singificantly lowered and the mixture is closer to combustion. Following the preconditioning, combustion may be initiated by irradiating the mixture with a superimposed ultraviolet flash of sufficient intensity to initiate combustion therein. Alternatively and as another example, the preconditioning of the mixture may be followed by a superimposed electrical spark discharge therein or by some other heating method to initiate combustion.
The present invention also utilizes the irradiation of a combustible mixture from a plurality of sources of ultraviolet energy, thereby enhancing the photochemical combustion process. By the use of such plural sources, it has been found that different zones within a combustible mixture may be preconditioned and the combustion processes therein controlled to enhance the combustion process of the overall mixture.
The invention contemplates unique sources of ultraviolet energy in the form of spaced electrodes in an inert gas atmosphere. In one embodiment a window is employed to provide for the transmission of ultraviolet energy from a sealed source, without appreciable absorption thereof, into a combustible mixture. In another embodiment, a 37 windowless" source is employed utilizing a flow of inert gas to dynamically create the necessary gaseous conditions between the electrodes, displacing foreign gas, and thus providing for efficient generation of the necessary ultraviolet radiation.
The present invention thus has application to all combustion processes, and in particular to those combustion processes involving oxygen, e.g., combustion processes in automotive and aircraft engines. The techniques may be employed in enhancing combustion in all combustion chambers, including exhaust systems of combustion devices.
The work to date in the investigation of photochemical control of the combustion process has largely been theoretical. Representative publications are as follows:
1. "Final report -- Photochemical Enhancement of Combustion and Mixing in Supersonic Flows, " by A. E. Cerkanowicz, Photochem Industries, Inc., Fairfield, N.J., dated Nov. 1973, distributed on Apr. 1, 1974. PA1 2. "Interim Scientific Report -- Photochemical Enhancement of Combustion and Mixing in Supersonic Flows," by A. E. Cerkanowicz, Photochem Industries., Fairfield, N.J., dated March 1972, distributed on May 16, 1973. PA1 3. "Photochemical Ignition and Combustion Enhancement in High Speed Flows of Fuel-Air Mixtures," by A. E. Cerkanowicz and R. F. McAlevy III, Photochem Industries, Incorporated, Fairfield, New Jersey, published by American Institute of Aeronautics and Astronautics at AIAA 11TH AEROSPACE SCIENCES MEETING, WASHINGTON, D.C./Jan. 1-12, 1973, AIAA Paper No. 73-216. PA1 7. "Photochemical Ignition of Low Pressure Fuel-Oxidizer Mixtures," by M. E. Levy and A. E. Cerkanowicz, Vitro Laboratories, West Orange, New Jersey and R. F. McAlevy III, Mechanical Engineering Department, Stevens Institute of Technology, Hoboken, New Jersey, paper delivered before fall meeting of the Western States Section of the Combustion Institute held at Stamford Research Center Institute, Palo Alto, California (Monday and Tuesday of third week in October 1968). PA1 8. ROCKETS, October 1945, Page 10.
4. "The Photochemical Ignition Mechanism of Unsensitized Fuel-Air Mixtures," by A. E. Cerkanowicz, M. E. Levy and R. F. McAlevy III, Photochem Industries, Fairfield, New Jersey, published by American Institute of Aeronautics and Astronautics at AIAA 8TH AEROSPACE SCIENCES MEETING, NEW YORK, NEW YORK/Jan. 19-21, 1970, AIAA Paper No. 70-149.
5. "Argon Photoionization Cross-Sections and Autoionized Line Profiles in the 584-304 A Region," by M. E. Levy, Photochem Industries, Hoboken, N.J., 07030, and R. E. Huffman, Air Force Cambridge Research Laboratories, Bedford, Massachusetts 01731, published by Pergamon Press 1969 in J. QUANT, SPECTROSE, RADIAT, TRANSFER, Vol. 9, pp. 1349-1358, Printed in Great Britain.
6. "Ignition of Subatmospheric Gaseous Fuel-Oxidant Mixtures by Ultraviolet Irradiation," by M. E. Levy and A. E. Cerkanowicz, Vitro Laboratories, West Orange, New Jersey and R. F. McAlevy III, Stevens Institute of Technology, Hoboken, New Jersey, published by American Institute of Aeronautics and Astronautics at AIAA 7TH AEROSPACE SCIENCES MEETING, NEW YORK CITY, NEW YORK/Jan. 20-22, 1969, AIAA Paper No. 69-88.
Representative patents are as follows:
______________________________________ U.S. PATENT No. ISSUE DATE PATENTEE ______________________________________ 3,190,823 June 22, 1965 R. Bloxham 3,177,651 April 13, 1965 H. R. Lawrence 3,167,015 Jan. 26, 1965 B. Smith et al 3,049,874 Aug. 21, 1962 M. R. Morrow et al 3,122,887 March 3, 1964 B. J. Farmer British Patent No. 850,321 published 5 October 1960. ______________________________________
The invention will be more completely understood by reference to the following detailed description, which is to be read in conjunction with the appended drawings.